Utilization of recovered steam heat for heating the distillation zone



Feb. 6, 1968 K. D. UITTI ET AL UTILIZATION OF RECOVERED STEAM HEAT FOR HEATING THE DISTILLATION ZONE Filed Aug. 26, 1965 United States Patent Ofifice 3,367,846 UTILIZATION OF RECOVERED STEAM HEAT FOR HEATING THE DISTILLATION ZONE Kenneth D. Uitti, Bensenville, and Toshio Okuma, Des

Plaines, Ill., assignors to Universal Oil Products Company, Des Plaines, Ill., a corporation of Delaware Filed Aug. 26, 1965, Ser. No. 482,685 5 Claims. (Cl. 203-25) ABSTRACT 0F THE DISCLOSURE Multicolumn fractionation of a C-laromatics mixture for the recovery of benzene, toluene and ortho-xylene, wherein benzene is taken overhead in a first column, toluene is taken overhead in a second column, and the toluene column bottoms is separated in a third (orthoxylene) column to produce an ortho-xylene bottom fraction and a mixed light xylene overhead fraction. The ortho-xylene column overhead vapors are condensed in a steam generator and a portion of the resulting steam is used to reboil the benzene column.

This invention relates to the separation of a mixture of aromatic hydrocarbons by fractional distillation. More particularly, our invention is directed to a process for the recovery of ortho-xylene from a feedstock comprising a major proportion of benzene and/ or toluene and a minor proportion of xylene isomers including ortho-xylene, utllizing a multiple column fractionator train in which heat is abstracted from the column requiring the highest heat duty and used to reboil a preceding column having a lower reboiler temperature. Specifically, such heat recovery is effected by condensing the overhead vapors of the high heat duty column in indirect heat exchange, in a steam generator, with boiling water under a regulated, preferably superatmospheric, steam pressure; this column 1s further refiuxed hot whereby the steam generator provides precise automatic control of refiux temperature, in addition to furnishing steam for the lower temperature column..

A typical feedstock to the fractionation unit, which may comprise the product of an aromatics extraction plant, consists of 30%-47% benzene, 30%-47% toluene, 5 1% paraxylene, 15%-2.5% metaxylene, 5%-1% orthoxylene, %-1% ethylbenzene, 4%-0.5% C9 aromatics, and 1%0.05% diphenyl, all of the foregoing percentages being mol percentages. The fractionation unit is usually designed to recover substantially pure benzene, nitration grade toluene, and ortho-xylene of at least 95% purity.

A preferred fractionation scheme removes first the lowest boiling component, benzene, followed by toluene removal in a second column. These separations are relatively easy: for example, to recover a 100% benzene overhead fraction and a benzene-free bottoms fraction, the benzene column requires 45-60 trays, a heat flux of 20,000-33,200 B.t.u./lb.mol of feed, and an external reflux to feed mol ratio of from about 1:1 to about 2: 1; to recover a 100% toluene overhead fraction and a toluene free bottoms fraction, the toluene column requires 55- 70 trays, a heat flux of 212,200-28,400 B.t.u./lb.-Ino1 of feed, and an external reflux to feed m01 ratio of from about 1.00 to about 1.50.

The toluene column bottoms fraction, consisting of a major proportion of C8 aromatics and a minor proportion of C9 aromatics and diphenyl is then distilled in a third 3,367,846 Patented Feb. 6, 1968 column designed to effect a precise split between metaxylene and ortho-xylene. Light xylenes and ethylbenzene are taken overhead, while ortho-xylene, C9 aromatics and diphenyl constitute the bottoms fraction which is substantially free of light xylenes. This third separation is quite difficult, since the difference in boiling points of the key components in only 9 F, and must be carried out in a socalled superfractionator, designated herein as the orthoxylene column, requiring 1Z0-160 trays, a heat tiux of 79,000216,000 B.t.u/lb.mol of feed, and an external reflux to feed mol ratio of from about 5:1 to about 15:1. The utility requirements of the ortho-xylene column are relatively high; for example, at a feed rate of 5000 barrels per day, a water-cooled overhead condenser will require approximately 6000 gallons per minute of cooling water. In many areas of the world, water is in such scarce supply that quantities of this magnitude, even though circulated in a closed system, are wholly out of the question and it is necessary to use a forced convection air condenser. However, an air condenser for a highly refiuxed distillation column is unsatisfactory because the column is difficult to maintain in heat balance, refiux temperature being very sensitive to changes in ambient air temperature. The use of a compensating internal reflux controller is at best a compromise solution. And with either a water condenser or an air condenser, substantial quantities of heat are lost.

Our invention affords significant operating economies in the overall fractionation process as well as improved control of the ortho-xylene column itself. We have found that by appropriately adjusting the pressure of the orthoxylene column in relation to the bubble point ofthe benzene column bottoms, the ortho-xylene column overhead vapors can be condensed in indirect heat exchange with boiling waterin a steam generation zone, and the resulting steam is at a sufficiently high temperature level to reboil the benzene column bottoms by condensing such steam in indirect heat exchange therewith. Furthermore, the quantity of steam so generated exceeds that required to run the benzene column which generally has the highest heat duty of all columns except the ortho-xylene column; therefore, the total reboil heat of the benzene column can be supplied by the heat of condensation of the orthoxylene column overhead vapors. The excess steam generated by the ortho-xylene column overhead condenser permits independent control of steam pressure and temperature by a back-pressure regulator, and this in turn provides precise automatic stabilization of the orthoxylene column reflux temperature. The ortho-xylene column is refluxed hot, meaning that the condensed overhead fraction leaving the tube side of the steam generator is returned to the top of the column without any appreciable additional cooling. The refiux temperature will generally be at least 10 F. higher, and preferably 15 -50 F. higher, than the steam temperature depending upon design approach temperature of the condenser-steam generator.

One embodiment of our invention therefore provides a process for the separation of a hydrocarbon mixture comprising 'benzene and xylene isomers including ortho-xylene which comprises charging said mixture as feed to a first multistage distillation column; reboiling the first collumn bottoms -by indirect heat exchange in a reboiling zone with condensing steam obtained from a steam generation zone; recovering from the first column a benzenerich overhead fraction and a bottoms fraction which is substantially free of benzene; passing at least a portion of the first columns bottom fraction as feed to a second multistage distillation column operating under an overhead pressure in the range of -100 p.s.i.a.; passing the overhead vapors from the second column through said steam generation zone and therein condensing the overhead vapor by indirect heat exchange with boiling water under a regulated, preferably superatmospheric steam pressure and at a temperature higher than the temperature of said first column bottoms; recovering from the steam generation zone a condensed overhead fraction comprising meta-xylene or para-Xylene; recovering from the second column a bottoms fraction comprising orthoxylene; and withdrawing steam from the steam generation zone and passing at least a portion thereof to the reboiling zone of said first column.

The present invention is more particularly described in conjunction with the accompanying drawing which is a simplified schematic flow diagram of a three-column fractionator train embodying the principle of the invention. It will be understood that equipment such as pumps, valves, controls, miscellaneous heat recovery circuits, start-up lines, etc., which are omtted from the ow sheet, will be supplied as required in accordance with accepted practice.

With reference to the drawing, an aromatic feedstock comprising benzene, toluene, ortho-xylene, paraxylene and/ or metaxylene, together with minor amounts of ethylbenzene, C9 aromatics and diphenyl, is charged to benzene column 10 through line 11. Column 10 has 45-60 stages, which may be conventional bubble decks, sieve trays or valve trays. Overhead vapors are removed therefrom through line 12, totally condensed in air condenser 13, and drained to receiver 14. The liquid contents of receiver 14 are maintained under an atmosphere of suitable blanketing gas, such as sweet refinery gas, delivered from header 15 through line 16 at a regulated pressure. For vacuum operation, the blanketing gas would be omitted and line 16 would instead connect with a steam jet ejector or vacuum pump. Reiiux is returned to the column through line 17. Net overhead liquid, consisting essentially of 100% benzene, is removed from the column through a side-draw 18 several trays down from the top. Column 10 is reboiled by an external thermosyphon reboiler 19. Steam is supplied to the tubes of reboiler 19 from line 20 at a controlled rate of flow provided by flow controller 21 and motor valve 22. Condensed steam is drained from reboiler 19 through line 23 to a condensate receiver 24. Operating conditions for the benzene column are as follows:

When an air condenser is employed, which requires a relatively high approach temperature as against a watercooled condenser, the receiver pressure is desirably slightly above atmospheric, e.g. about 1 p.s.i.g. gas blanketing pressure. The overhead vapor condensation pressure is then in the range of 20-16 p.s.i.a. corresponding to a condensation temperature of 195 175 F.

The benzene column bottoms, substantially free of benzene, is charged as feed to toluene column through line 25. Column 30 has 55-70 stages which may be conventional bubble decks, sieve trays or valve trays. Overhead vapors are removed therefrom through line 32, totally condensed in air condenser 33 and drained to receiver 34. The liquid contents of receiver 34 are maintained under an atmosphere of blanketing gas delivered from header 15 to line 36. Reux is returned to column 30 via line 37, and net overhead liquid, consisting essentially of 100% toluene, is removed through line 38. Column 30 is reboiled by an yexternal thermosyphon reboiler 39. High pressure refinery steam is supplied to the tubes of re- 4 boiler 39 through line 40, and condensate is removed therefrom through line 43. Operating conditions for the toluene column are as follows:

When an air-condenser is employed, the receiver pressure is desirably slightly above atmosphere, e.g. about 1 p.s..i.g. gas blanketing pressure. The overhead vapor condensatlon pressure is then in the range of 20-16 p.s.i.a. corresponding to a condensation temperature of 250-235 F.

Toluene column bottoms, substantially free of toluene, is charged as feed through line 45 to ortho-xylene column 50. Column 50 has 1Z0-160 stages and, while diagramatically illustrated as a single column, is usually composed of two side-by-side columns connected in series, one column serving as the upper rectification section and the other being the lower stripping section. This column is designed and operated to separate ortho-xylene from meta-xylene or para-xylene; and net overhead product therefore comprises light xylenes and ethylbenzene, preferably containing less than about 1 mol percent of orthoxylene, and the bottoms product comprises ortho-xylene, C9 aromatics and diphenyl, preferably containing less than about 3 mol percent of light xylenes. Overhead vapors are removed through line 51 and are totally condensed in the tubes of steam generator 53. Column pressure is controlled by a pressure controller 55 actuating motor 52 in the net overhead line. The condensed overhead fraction is drained through line 51-a to a subjacent hot receiver 54 maintained at column pressure by an equalizing line 56. Hot reflux is returned to the column through line 57 and net overhead product is taken olf through line 58. Column 50 is reboiled by a forced circulation gas-fired or oil-fired tube furnace 59. The bottoms fraction, withdrawn through line 65, is passed to a finishing column (not illustrated) for heavy ends removal and recovery of 95% ortho-xylene.

Feedwater for steam generator 53 is supplied by line 71 from condensate receiver 24 to which make-up conf densate or treated deaerated feedwater is added via line 70. Saturated steam is withdrawn from the generator through line 74, a portion thereof being diverted through line 20 to run the benzene column reboiler 19 and the re` mainder being discharged through valve 76 and line 77 for other uses such as dri-ving turbines or deaerating boiler feedwater. Rate of feedwater addition is controlled by a conventional three element boiler feedwater computer actuating a motor valve 73. Information transmitted to computer 80 consists of feedwater rate from flowmeter 72, steam demand from iiowmeter 75 and drum level from level transmitter 79. Steam generation pressure is controlled by a pressure controller 78 actuating motor valve 76 in the net drawoff line. This fixes the shell side temperature of generator 53 and affords precise regulation of reflux temperature for column 50. As previously indicated, the steam temperature is at least 10 F. lower, and preferably 15-50 F. lower, than the reux temperature in order to provide optimum mean temperature difference. The overhead vapor pressure of column 50 is so established in relation to the bubble point of the benzene column bottoms that the steam supplied to reboiler 19 is at a usefully high temperature; in general, thesaturated steam temperature should be at least 5 F, higher, preferably of the order of 20-60 F. higher, than the benzene column bottoms temperature. As is well known, as distillation pressure is increased, the relative volatilities of the feed components decrease so that the column will require a higher reflux ratio and/or an increased number of trays to effect a specified degree of separation. For the mixed xylene system here involved, the effect of higher distillation pressure does not become significantly adverse until such pressure exceeds about 100 p.s.i.a. Accordingly, the overhead pressure of ortho-Xylene column 50 will be maintained, in general, in the range of -100 p.s.i.a. dependxylene isomers including ortho-xylene which comprises:

(1) Charging said mixture as feed to a first multistage distillation column;

(2) Reboiling the first column bottoms in a reboiling ing upon the pressure of the benzene column. It is pre- 5 zone by indirect heat exchange with condensing ferred that the ortho-xylene column pressure, as measured steam; in the overhead vapor line, be in the range of 30-75 p.s.i.a. (3) Recovering from said first column a benzene-rich It will be appreciated that our preferred pressures are overhead fraction and a bottoms fraction substansubstantially in excess of those heretofore employed in the tially free of benzene; fractionation of mixed Xylenes, usually of the order of lo (4) Charging said first column bottoms fraction as feed 18 to 25 p.s.i.a. according to previous practice and proto a second multistage distillation column; cedure. (5 Recovering from said second column a toluene-rich Operating conditions for the ortho-xylene column are overhead fraction and a bottoms fraction substanas follows: tially free of toluene;

(6) Charging said second column bottoms fraction as Broad Preferred feed to a third multistage distillation column operating under an overhead pressure in the range of 10- Overhead pressure, p.s.i.a 10400 30-75 100 p.s.i.a.; gggllegnfglgel gjg 332:33 (7) Passing the overhead vapors from said third colggortsedpresiuep le 423258 umn through a steam generation zone and therein Bottoms gempera'e; 34(1 500 405475 condensing said overhead vapors by indirect heat ex- Refiux tofeed m01rati0- 5-15 7-10 change with boiling water under controlled pressure and at a temperature at least 5 F. higher than the The practice of our invention is further illustrated by temperature of said first column bottoms; the following specific example in which an aromatic feed- (8) Recovering from said steam generation zone a constock at the rate of approximately 20,000 b.p.d. is procdensed overhead fraction comprising meta-xylene or essed using the flowscheme of the drawing. A material para-xylene and returning a portion of said conbalance for the three distillation columns appears in Table densed overhead fraction, without appreciable addi- I while operating conditions for each of the columns is tional cooling thereof, to said third column as refiux; given in Table II.

TABLE I Benzene Col. Benxene Col. Benzene Col. Toluene Col, Toluene Col. o-Xylene Col.. o-Xylene Col, Feed (line 11) Ovhd. (line 18) Betts. (line 25) Ovhd. (line 38) Botts. (line 45) Ovhd. (line 5B) Botts. (line 65) Mol, Mol, Mol, Mols/hr. Percent Mols/hr. Percent Mols/hr. Percent Mol, Mols/hr. Percent :BenzeneA 1, 160 Toluene... 1, 150 Et. Ben- 106` ZBIle. p-X ylene 94. 2 mXylene. 237 o-Xylene.- 89. 2 CtAro- 57.2

matics. Di-phenyl 6.9

TABLE II B T l O th l (9) Recovering from said third column a bottoms frac- Cgl ofglgne tion comprising ortho-xylene;

(10) Maintaining a steam pressure controlled zone 1n Notrays 54 62 140 50 open Huid communication with said steam generatlon Overhead pressure, p.s.i.a 20 20 40 Zone; and 03%???ilffffffr 195 250 365 (1l) Withdrawing saturated steam from said steam egux lt;@Irllrrmgturle-t1-A 7 12(2)? g generation zone and passing one portion thereof to Bttsoprgsupffg:fr: 5 '25 '56 the reboiling zone of said rst column as aforesaid, Bottoms temperature,F- 250 350 425 55 and passing the remaining of said saturated steam The ortho-xylene column overhead condenser steam generator produces 97,400 lbs. per hour of saturated steam at 75 p.s.i.a. for a duty of 88,000,000 B.t.u. per hour; this represents a recovery of more than 99% of the heat input to the column supplied by the fired reboiler. Of this steam, 62,000 lbs. per hour are passed to and condensed in the benzene column reboiler which has a duty of 56.5 million B.t.u. per hour. By contrast, if the benzene column reboiler were run by normal refinery high pressure steam, at, say, 250 p.s.i.g. pressure, it would require approximately 69,000 lbs. per hour of such steam. Our invention therefore saves 69,000 lbs. per hour of refinery steam, in addition to supplying over 35,000 lbs. per hour of low pressure steam for other uses and additionally provides improved stabilized operation of the ortho-xylene column while eliminating an air condenser or a water condenser.

We claim as our invention:

1. Process for the separation by fractional distillation of a hydrocarbon mixture comprising benzene; toluene and through said pressure controlled zone to discharge.

2. Process of claim 3 wherein said third distillation column is operated under an overhead pressure in the range of 30-75 p.s.i.a.

3. Process for the separation by fractional distillation of an aromatic hydrocarbon mixture comprising benzene, toluene and xylene isomers including ortho-xylene which comprises:

(1) Charging said mixture as feed to a first multistage distillation column;

(2) Reboiling the first column bottoms in a reboiling zone by indirect heat exchange with condensing steam constituting the sole source of reboiling heat for said first column;

(3) Recovering from said first column a benzene-rich overhead fraction and a bottoms fraction substantially free of benzene;

(4) Charging said first column bottoms fraction as feed to a second multistage distillation column;

(5 Recovering from said second column a toluene-rich y(7,) Passing the overhead vapors from said third column through a steam generation zone and therein condensing said overhead vapors by indirect heat eX- change with boiling water under controlled pressure and at a temperature at least 5 F. higher than the temperature of said rst column bottoms;

(8) Recovering from said steam generation zone a condensed overhead fraction comprising meta-Xylene or para-Xylene and returning a portion of said condensed overhead fraction, without appreciable additional cooling thereof, to said third column as reflux;

(9) Recovering from said third column a bottoms fraction comprising ortho-Xylene;

(10) Maintaining a steam pressure controlled zone in open uid communication with said steam generation zone; and

`(11) Withdrawing saturated steam from said steam generation zone and passing one portion thereof to the reboiling -zone of said rst column as aforesaid, and passing the remaining portion of said saturated y8 steam through the pressure controlled zone to discharge.

4. Process of claim 3 wherein the reboiler heat to said third distillation column is supplied by indirect heat ex- -5 change with the combustion products of a burning fuel.

5. Process of claim 3 wherein said steam generation temperature is atleast 10 F. lower than the temperature ofthe condensed overhead fraction leaving said steam generation zone.

,10 References Cited UNITED STATES PATENTS Re. 22,379 9/1943 Dunn et a1 203-56 1,874,387 8/1932 Torrey et al. 208-354 X 1,882,568 10/1932 Hau 20s- 354 15 2,325,379 7/1943 Durrum 20s-57 2,406,695 s/1946 Lake 20s-56 2,577,701 12/1951 Deming et a1 20s-25 X 3,254,024 5/1966 Huckins et a1 20327 X 20 3,265,590 8/1966 Redelay 203-21 FOREIGN PATENTS 912,978 8/1946 France.

NORMAN YUDKOFF, Primary Examiner. '25 F. E. DRUMMOND, Assistant Examiner. 

